linux-kernel - Re: [PATCH] mmc: mmc: Fix the timing for clock changing in mmc

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Message-ID: <CAPDyKFr9H2XcgCk9AmHgJfHC+PySh66KxegMJ4yb4aqKSVt3kg@mail.gmail.com>
Date:   Wed, 4 Mar 2020 12:08:59 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Kyungmin Seo <kyungmin.seo@...el.com>
Cc:     "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mmc: mmc: Fix the timing for clock changing in mmc

On Wed, 12 Feb 2020 at 03:40, Kyungmin Seo <kyungmin.seo@...el.com> wrote:
>
> The clock has to be changed after sending CMD6 for HS mode selection in
> mmc_hs400_to_hs200() function.
>
> The JEDEC 5.0 and 5.1 said that "High-speed" mode selection has to
> enable the the high speed mode timing in the Device, before chaning the
> clock frequency to a frequency between 26MHz and 52MHz.

I think that is based upon the assumption that you are using a lower
frequency to start with.

For example, assume that you are running with 400KHz during card
initialization, then you want to send the CMD6 to switch to HS mode
and that should be done, before updating the clock rate.

mmc_hs400_to_hs200() goes the opposite direction, so I think the
current code looks correct to me.

Kind regards
Uffe

>
> Signed-off-by: Kyungmin Seo <kyungmin.seo@...el.com>
> ---
>  drivers/mmc/core/mmc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mmc/core/mmc.c b/drivers/mmc/core/mmc.c
> index 3486bc7fbb64..98640b51c73e 100644
> --- a/drivers/mmc/core/mmc.c
> +++ b/drivers/mmc/core/mmc.c
> @@ -1196,10 +1196,6 @@ int mmc_hs400_to_hs200(struct mmc_card *card)
>         int err;
>         u8 val;
>
> -       /* Reduce frequency to HS */
> -       max_dtr = card->ext_csd.hs_max_dtr;
> -       mmc_set_clock(host, max_dtr);
> -
>         /* Switch HS400 to HS DDR */
>         val = EXT_CSD_TIMING_HS;
>         err = __mmc_switch(card, EXT_CSD_CMD_SET_NORMAL, EXT_CSD_HS_TIMING,
> @@ -1210,6 +1206,10 @@ int mmc_hs400_to_hs200(struct mmc_card *card)
>
>         mmc_set_timing(host, MMC_TIMING_MMC_DDR52);
>
> +       /* Reduce frequency to HS */
> +       max_dtr = card->ext_csd.hs_max_dtr;
> +       mmc_set_clock(host, max_dtr);
> +
>         err = mmc_switch_status(card);
>         if (err)
>                 goto out_err;
> --
> 2.17.1
>